The present invention relates to a process for making silica bricks with high bulk density from a starting mix with graded granular quartzite (silica) and elemental silicon, and to the bricks obtained by this process.
Such silica bricks, which are also known as silicon dioxide bricks, are used in blast furnaces, hot-blast stoves and especially coke ovens, and an attempt has been made to achieve a bulk density as high as possible of such silica bricks so as to improve their thermal conductivity.
West German Patent No. 2,836,691 describes a process for making such silica bricks with high bulk density, and in this case 0.5 to 10 weight percent of silicon nitride and/or silicon carbide is added to the starting mix which is then treated in a temperature range between 1200 and 1400.degree. C. under very special conditions relating to the oxygen concentration below the gas outlet opening in the firing kiln and the rate of temperature rise in .degree. C./hr. This West German Patent No. 2,836,691 also indicates, as a teaching of the prior art, that silica bricks can also be made with admixture of metal oxides, such as Cu.sub.2 O, TiO.sub.2 and Fe.sub.2 O.sub.3 or by addition of silicon carbide.
In order to obtain silica bricks with low porosity, and hence with improved corrosion resistance, the use of elemental silicon or silicon carbide together with an ammonium compound or especially a nonvolatile oxidizing compound, such as calcium or magnesium nitrate, is known in the art of making silica bricks (British Patent Nos. 880,582 and 1,012,363). The use of water-soluble compounds for preparing the mix is disadvantageous, however, and difficulties occur during the firing of the bricks. There is also the risk of additional environmental pollution because of the use of salt-like compounds.
U.S. Pat. No. 3,144,345 discloses a process for making silica bricks with high density and high thermal conductivity, in which finely divided amorphous silicic acid (condensed from the vapor phase) is employed in the mix. The resulting silica bricks attain a density of up to 1.84 g/cm.sup.3 (115 lb/ft.sup.3), as apparent from column 4, Table I, Example 4.